Any afr gauge that does anything good for you is a wideband, and they are pretty expensive.
an air to fuel ratio gauge just has flashy lights and doesnt really tell you anything.
The AEM uego is probably the cheapest one, IVe seen it for as low as 230, but I am going to shoot for the innovate lc-1 kit. They have it paired up with a DB gauge(I think it would match the prosport gauges nicely, and its like 239 or so.
Both are pretty good, both are very accurate, but the wideband shootout post has information saying the aem has a latency problem(which means the afr is displayed a little late)